1. Field of the Invention (Technical Field)
The present invention relates to method and apparatuses for determining velocities of objects.
2. Description of Related Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art via-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Velocity versus time measurement is important to laser-produced high-energy density plasma experiments because the transient motion of the surface, which depends on both the equation of the state of the surface material and laser-produced plasma, usually requires submicrosecond time resolution and large dynamic range for velocity measurement. Two examples of laser-produced plasma experiments are shown in FIGS. 1(a) and 1(b). See D. L. Paisley, Shock Compression of Condensed Matter, Albuquerque, N.Mex., August 1989, edited by S. C. Schmidt, et al., Elsevier Science, Amsterdam, (1990); R. H. Warnes, et al., AIP Conf. Proc. 370, 495 (1996); and S. Watson, et al., J. Phys. D 33, 170 (2000). These setups can accelerate metal surfaces to high velocities (0.1 to >5 km/s). In FIG. 1(a), the plasma is confined between a transparent substrate and the metal flyer plate to be accelerated. Keeping the laser power below the optical breakdown threshold of the substrate, typically 1 GW/cm2, the laser beam is transmitted through the substrate, depositing energy at the ablative layer between the substrate and the flyer plate. The pressure in the confined plasma expands, accelerating the flyer plate away from the transparent substrate, FIG. 1(b) is for higher laser power, 1012-1015 W/cm2, and lower coupling efficiency. The expanding ablative plasma from one side will accelerate the remaining thickness of the foil/film to a high velocity. Although not as efficient, higher velocities can usually be obtained by this method than that in FIG. 1(a).
One of the most accurate methods for velocity versus time measurement is velocity interferometer system for any reflector (VISAR), L. M. Barker, et al., J. Appl. Phys. 43, 4669 (1972). In VISAR, the Doppler-shifted light from the scattering surface recombines with a time-delayed copy to form an interference pattern with fringes, from which Doppler shift and velocity are obtained. Time evolution of the interference fringes gives the history of velocity of the reflecting surface. Some of the advantages of VISAR are that counting fringes leads to a large dynamic range, and that the built-in equality of light levels in the two interference legs leads to robust contrast. Some of the disadvantages are missed fringes, expense, large amounts of light needed for multiple points, and difficulty in handling multiple frequencies that correspond to different velocities of the reflecting surface. In addition, determining the accuracy and precision of measurements using a VISAR is not straightforward and involves the choice of the velocity-per-fringe (VPF), shock rise time, and temporal resolution of the instrument recording the VISAR raw signals, as well as the quality of the recorded signal. The accuracy and resolution of a given experimental record may not be, and usually is not, consistent over the entire recording time. The accuracy of the recorded signals is ultimately limited by the recording resolution of the Lissajous angle, the phase angle between the sine and cosine of VISAR signals. L. M. Barker, AIP Conf. Proc. 429, 833 (1998). All of the above listed parameters fold together to determine the precision with which data are recorded. Barker and Hollenhach, L. M. Barker, et al., J. Appl. Phys. 45, 3692 (1974), and others, W. F. Hemsing, Rev. Sci. Instrum. 50, 73 (1979), D. D. Bloomquist, et al., Proc. SPIE 348, 523 (1982), addressed the resolution, accuracy, and precision of VISAR data. A value of 1%-3% accuracy and precision is generally regarded as “typical” for good quality of recorded signals, but can be better under “the most ideal” circumstances.
Laser Doppler velocimetry (LDV) is a technique for making noninvasive measurements of surface motion or fluid flow. A Doppler frequency shift ΔfD is proportional to the velocity along the reflected beam from the moving target. The frequency shift is measured electronically by mixing the light returning from the target with a reference beam. This system was known to have ambiguity in its measurement of the direction of the motion. Photonic Doppler velocimetry (PDV) or heterodyne velocimeter (HetV), O. T. Strand, et al., Proceedings of the 26th International Congress on High-Speed Photography Photonics, Alexandria, Va. (2004), has recently been demonstrated for velocities up to 3.6 km/s using a 1.55 μm laser, fast digitizers, and other electronics. The velocity of an object is proportional to the b eat frequency of the incident and reflected laser in PDV. For 1 km/s, the beat frequency that must be digitized is 1.29 GHz.
The present invention provides an alternative laser-based technique called correlated-intensity velocimeter for arbitrary reflector (CIVAR), which can also be used for velocity measurement of reflecting surfaces in real time in laser-produced plasma experiments. The invention is described in Z. Wang, et al., “Correlated-Intensity Velocimeter for Arbitrary Reflector for Laser-Produced Plasma Experiments”, Rev. Sci. Instrum. 77, 10E516 (2006).